Excluding One of Darwin's Postulates: What Would Happen?

[immunogoblin]

Hi everyone,
We know that the result of meeting all four of Darwin's postulates is evolution by natural selection. So I got to wondering...

If, in a theoretical population, the first 3 of Darwin's postulates were met, but instead of "4. Survival and reproduction of individuals are not random. " a population undergoes random reproductive success... what would happen? You wouldn't get evolution, that's for sure.

Here are the postulates as I know them:

1. Individual variation (i.e. variation in traits).
2. Phenotypic variation is passed onto offspring (i.e. traits are heritable).
3. In every generation, more progeny are produced than can survive.
4. Survival and reproduction of individuals are not random.

[JorgeLobo]

Darwin describes - does not dictate. Evolution as we understand it would exist wth or without Darwin.

[AstraSequi]

You would still get some random changes in allele frequencies (genetic drift). The population in question would soon become extinct though - it couldn't adapt to environmental changes, and couldn't suppress the harmful variations that arise.

But yeah, like JorgeLobo said - the postulates are a description of what is observed to happen, rather than having any influence on what does happen.

[immunogoblin]

@JorgeLobo: For sure, I definitely didn't mean to imply that. Need to work on my precision in writing!

@AstraSequi: Genetic drift! Of course! Why on earth did I forget about that?!?

Thank you both for the insight.

Cheers,
Ig

[immunogoblin]

Speaking of genetic drift, I've got another question:
Can genetic drift cause adaptive evolution?

A presentation I'm reading attributes adaptive evolution to the mechanism of natural selection and non-adaptive (or neutral) evolution to genetic drift. It says that drift can lead to non-adaptive evolution by reducing genetic variability within a population and causing fixation or extinction or unfavourable and favourable traits respectively.

Isn't drift random by definition and able to randomly lead to evolution that is adaptive? Or am I missing something?